I got stuck fairly early. I tried the site's Play option for the first time to get a hint. It told me to play 5 at (R8,C1) but I don't understand the logic to the move. I had located my 5s in row 7 at either (R7,C2) or (R7,C7) and my 5s in row 8 at either (R8,C1) (the hint) or (R8,C7). Can anybody explain why the hint is a logical move?

In addition to the naked pair mentioned by keith in row 9, there is also one in column 5. When the exclusions for both these are made, that leads to the other pair mentioned by keith the one in row 8. Making the exclusions from all the pairs leaves 5 as the only possibility for r8c1.